The observation of magnetization in an object or subject of interest disposed in an external magnetic field can provide valuable information on various phenomena of interest.
For example, the human body contains mainly diamagnetic materials with magnetic volume susceptibilities up to about χ=−10 ppm (Schenck, Med. Phys. 23(6):815-850 (1996)). When placed in a magnetic field with a magnetic field intensity B0, e.g. in a magnetic resonance (MR) imaging set-up, the magnetized body and its parts cause field changes up to the order of χB0. At the body's surface and in the surrounding volume these effects typically drop to a small fraction of this value. However, in MR imaging magnets of typically between 1.0 T and 10.0 T, field fluctuations due, e.g., to physiological motion such as breathing reach amplitudes up to microtesla range outside the body. Heart motion and pulsatile blood flow equally cause magnetic field perturbations outside the chest. These indirectly reflect cardiac function and are hence interesting to observe. Dynamic variation of susceptibility effects can also occur due to physical, chemical or biological processes that change matter at the atomic, molecular or macroscopic level. For instance, the oxygenation and deoxygenation of blood change the magnetic properties of the contained hemoglobin. Observing such effects is interesting not only in the human body, animals or plants. It is also interesting for inanimate material samples as well as chemical and biological samples, including cell cultures and samples of body liquids.
Magnetic susceptibility is generally dominated by the magnetism of the electrons contained in a sample. However, another smaller susceptibility contribution also arises from atomic nuclei with non-zero spin such as 1H, 2H, 13C, 17O, 19F, 23Na and 31P. The related nuclear magnetization is the basis of nuclear magnetic resonance (NMR) and its imaging variant, magnetic resonance imaging (MRI). Like electronic magnetization, in the thermal equilibrium nuclear magnetization is aligned with the background magnetic field, which conventionally defines the z axis of an NMR or MRI system.
In NMR processes, nuclear magnetization is observed by flipping it away from the z axis and observing the resulting transverse magnetization component based on it precession about the z axis. In this way, NMR and MRI measures nuclear magnetization. However, the actually observed physical quantity is its transverse component Mxy. It would be interesting to also observe the z component of nuclear magnetization, Mz. For instance, for measuring (distributions of) the longitudinal relaxation time, T1, this would be very beneficial. The value of Mz at a given time can be inferred by controlled flipping of the magnetization into the xy-plane and subsequent observation of the oscillating xy-signal. However, this approach only allows for snapshot information about Mz at a particular point in time and not for substantially continuous monitoring. Direct observation of Mz would solve this problem. Generally, the capability of observing not only Mxy but also Mz would add to the insight that can be gained in NMR and MRI experiments.
So for both types of magnetization, electronic and nuclear, one would like to observe Mz, i.e., the component parallel to the background field, by direct measurement.
NMR-based magnetic field probes have recently been used for monitoring fields and potential field perturbations generated by components of MR systems, including main magnets, gradients, and shim coils (Barmet et al. MRM 60:187-197 (2008)). At the temporal resolutions required for this purpose, i.e. at sampling rates of 1 kHz or higher, current probes offer field resolution in the order of microteslas (De Zanche et al. MRM 60:176-186 (2008)). However, for many applications, particularly for Mz determinations as mentioned hereinabove, it would be desirable to achieve even higher field resolution.
A magnetocardiography method by means of an external magnetometer based on magnetic resonance has previously been described (S. Groeger et al. Sensors and Actuators A 129: 1-5 (2006). However, this method relies on a continuous detection of an optical anisotropy in laser-pumped Cesium vapor.